A conventional manner of achieving "no tie down" characteristics has been to arrange a system of valves controlled by a pulse valve. This system or circuit insures a pressurized output can be obtained only if two hand valves are actuated within a pre-determined small time interval, and held continuously during the hazardous operation. This procedure has obvious beneficial application in controlling hazardous machinery, since proper mounting of the hand valves can make it impossible for the operator thereof to have either of his hands in dangerous areas during actuation of the machinery. Apart from the desirability of this type of safety device, it is now legally required that any hazardous industrial machinery in the United States be equipped with such a safety system.
However, there are certain problems with prior art types of "no tie down" systems. Many of them are extremely complex resulting in high cost and increased probability of failure. Moreover, some of them are vulnerable to circumvention by operators opposed to their use.